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During the most recent glacial advance a 2000 foot wall of ice near Missoula, Montana, prevented melt water from draining to the Pacific. A tremendous lake backed up into several nearby river valleys.

The lake is best-known for its periodic releases: when the ice dam broke, as it did many times before reforming again, it would discharge a flow of water dozens of times larger than the largest river in the world today.

The same topography that permitted the formation of this lake is responsible for a ghostly and analogous effect that is visible every winter. Ground inversions are common in the five valleys surrounding Missoula. Cold air, which flows down from the surrounding mountain ranges, settles behind the natural bottlenecks that once held water. The weather can become beautifully spring-like above but still not mix with the inverted cold.

Over five days we spent in Missoula in December, the inversion was fierce. Thick, feathery hoar frost covered the branches of trees in town, building up day after day. But in the mountains we could find temperatures that were almost 30 degrees warmer. At the end of a day spent skiing or hiking, we’d have to stop and put on sweaters and coats and hats and gloves before reentering the frozen world trapped beneath the layer of clouds.

One day Jen pointed out that the inversion was sitting at an elevation that fit right in with the horizontal bands marking the ancient lake shores of Lake Missoula, thousands of feet above the valley floor. So it was. This is what it was like looking out across the frozen inland sea that existed 12,000 years ago, when the first humans began crossing the Bering Land Bridge to North America.